Catalyst and process for preparing color-reduced polyisocyanates containing isocyanurate groups

ABSTRACT

The present invention relates to a catalyst and to a process for preparing color-reduced polyisocyanates containing isocyanurate groups.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst and to a process forpreparing color-reduced polyisocyanates containing isocyanurate groups.

2. Discussion of the Background

For high-quality one- and two-component polyurethane coating materialspossessing good light and weathering stability, polyisocyanate mixturescontaining isocyanurate groups and uretdione groups are typically usedas the isocyanate component.

For the preparation of polyisocyanates containing isocyanurate groupsand uretdione groups, which are suitable as raw materials orpolyurethane coating formulations, a variety of processes are known.These processes typically differ in the selection of the trimerizationcatalysts or in the selection of the organic isocyanates to be used inthe oligomerization reaction (cf., e.g., GB 1 391 066, EP-A-0 082 987,DE-A 39 02 078, EP-A-0 339 396, EP-A-0 224 165).

Suitable isocyanates used for trimerization, examples of which includearomatic, cycloaliphatic and aliphatic polyisocyanates having afunctionality of two or more, can be prepared by various kinds ofprocesses (Annalen der Chemie 562 (1949) pages 75 ff.). Those which haveproven particularly suitable in industry include preparation byphosgenation of organic polyamines to the corresponding polycarbamoylchlorides and the thermal cleavage of the chlorides into organicpolyisocyanates and hydrogen chloride. Alternatively, organicpolyisocyanates can be prepared without the use of phosgene, i.e., byphosgene-free processes. According to EP-A-0 126 299 (U.S. Pat. No.4,596,678), EP-A-126 300 (U.S. Pat. No. 4,596,679) and EP-A-355 443(U.S. Pat. No. 5,087,739), for example, (cyclo)aliphaticdiisocyanates—such as 1,6-hexa-methylenediisocyanate (HDI) and/orisomeric aliphatic diisocyanates having 6 carbon atoms in the alkyleneradical, and 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane(isophorone diisocyanate or IPDI)—can be prepared by reacting(cyclo)aliphatic diamines with urea and alcohols to give(cyclo)aliphatic biscarbamic esters and thermally cleaving these estersinto the corresponding diisocyanates and alcohols. The synthesis takesplace continuously in a circulation process and in the presence, ifdesired, of N-unsubstituted carbamic esters, dialkyl carbonates, andother byproducts returned from the reaction process.

Examples of catalysts which can be used for the trimerization ofisocyanates to give the desired polyisocyanates containing isocyanurategroups and uretdione groups include tertiary amines, phosphines, alkalimetal phenoxides, aminosilanes, quaternary ammonium hydroxides, andquaternary ammonium carbonates. Highly suitable oligomerizationcatalysts are hydroxides, halides or carboxylates ofhydroxyalkylammonium ions (cf., e.g. EP-A-0 351 873, U.S. Pat. No.5,290,902), alkali metal salts, and tin salts, zinc salts and lead saltsof alkylcarboxylic acids. Depending on the catalyst, it is also possibleto use various cocatalysts such as, for example, OH-functionalizedcompounds or Mannich bases comprising secondary amines and aldehydesand/or ketones.

For the oligomerization, the (cyclo)aliphatic diisocyanates are reactedin the presence of the catalyst, with or without the use as solventsand/or auxiliaries, until the desired conversion is attained. Partialtrimerization is one of the terms used in this context, since the targetconversion is generally well below 100%. Subsequently, the reaction isterminated by deactivating the catalyst and the excess monomericdiisocyanate is usually separated off, generally by flash distillationor thin-film distillation. Deactivation is carried out thermally or byadding a catalyst inhibitor such as, for example, p-toluenesulfonic acidor bis(2-ethylhexyl) phosphate. Particularly advantageous in the contextof the trimerization of isocycanates on the industrial scale is the useof quaternary hydroxyalkylammonium carboxylates as oligomerizationcatalysts. These catalysts of the choline type are thermally unstable.It is unnecessary to terminate the trimerization on reaching the desiredconversion by adding catalyst inhibitors which have the potential toreduce the quality. Instead, the controlled thermal deactivation permitsoptimum process control. The thermal instability is also advantageousfrom the standpoint of process safety. Uncontrolled “runaway” of thereaction is impossible, provided the amount of catalyst metered inremains below a multiple of the usual amount.

Depending on the type of catalyst used and the reaction temperature, theresulting polyisocyanates have different proportions of isocyanurategroups and/or uretdione groups. The products are usually clear, althoughproducts with a more or less strong yellow coloration may also beobtained depending on the type of catalyst, quality of diisocyanate,temperature of reaction and reaction regime. For the preparation ofhigh-quality polyurethane coating materials, however, products having avery low color number are desired.

The unwanted yellow also occurs when the otherwise highly advantageousquaternary hydroxyalkylammonium carboxylates are used (vide supra), sothat there is a specific requirement for improvement in this context.Surprisingly, it has now been found that, in comparison to othercatalysts of this type, specific quaternary hydroxyalkylammoniumcarboxylates provide polyisocyanates, which contain isocyanurate groups,having markedly improved color quality.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forpreparing polyisocyates that avoids the problems described above.

This and other objects of the invention have been achieved by thepresent invention, the first embodiment of which provides a process forpreparing color-reduced polyisocyanates containing isocyanurate groups,which process includes:

trimerizing at least one diisocyanate in the presence of 0.04-2% byweight, based on the weight of the diisocyanate, at least onetrimerization catalyst of the formula (I):

 wherein

 and wherein:

A, B, C, D and E independently of one another or simultaneously arehydrogen, chloro, C1-C4 alkyl, C1-C4 alkoxy, C1-C4 alkoxycarbonyl,hydroxyl, (R⁵)₃SiO—, (R⁵)₂N—, —COOH, (R⁵)₂N—CH₂— or phenyl, it beingpossible for any two adjacent A, B, C, D and E radicals to form aconjoint 5- or 6-membered saturated or unsaturated ring which mayoptionally and additionally include N, S or O as heteroatom;

F is hydrogen or methyl;

R² and R³ independently of one another or simultaneously are C1-C4alkyl, C2-C6 hydroxyalkyl with the hydroxyl group in position 2 relativeto the quaternary nitrogen or R¹;

R⁴ is hydrogen, methyl, C2-C10 alkyl, C3-C8 cycloalkyl or C2-C12 alkoxy;

R⁵ is C1-C4 alkyl;

Y⁻ is R⁶COO⁻ or OH⁻; and

R⁶ is hydrogen or a branched or unbranched, aliphatic or araliphatic,C1-C10 alkyl radical.

Another embodiment of the invention provides a trimerization catalyst ofthe formula (I):

wherein

and where the variables are defined as follows:

A, B, C, D and E independently of one another or simultaneously arehydrogen, chloro, C1-C4 alkyl, C1-C4 alkoxy, C1-C4 alkoxycarbonyl,hydroxyl, (R⁵)₃SiO—, (R⁵)₂N—, —COOH, (R⁵)₂N—CH₂— or phenyl, it beingpossible for any two adjacent A, B, C, D and E radicals to form aconjoint 5- or 6-membered saturated or unsaturated ring which mayoptionally and additionally include N, S or O as heteroatom;

F is hydrogen or methyl;

R² and R³ independently of one another or simultaneously are C1-C4alkyl, C2-C6 hydroxyalkyl with a hydroxyl group in position 2 relativeto the quaternary nitrogen in formula (I), or R¹;

R⁴ is hydrogen, methyl, C2-C10 alkyl, C3-C8 cycloalkyl or C2-C12 alkoxy;

R⁵ is C1-C4 alkyl;

Y⁻ is R⁶COO⁻ or OH⁻; and

R⁶ is hydrogen or a branched or unbranched, aliphatic or araliphatic,C1-C10 alkyl radical.

DETAILED DESCRIPTION OF THE INVENTION

Various other objects, features and attendant advantages of the presentinvention will be more fully appreciated as the same becomes betterunderstood from the following detailed description of the preferredembodiments of the invention.

Preferably, the invention provides a process for preparing color-reducedpolyisocyanates containing isocyanurate groups by partially trimerizingaliphatic, cycloaliphatic and/or (cyclo)aliphatic diisocyanates andsubsequently separating off excess diisocyanate, which includesperforming the trimerization in the presence of 0.04-2% by weight, basedon the weight of the diisocyanate used, of at least one trimerizationcatalyst of the general formula (I)

where

and where the variables are defined as follows:

A, B, C, D and E independently of one another or simultaneously arehydrogen, chloro, C1-C4 alkyl, C1-C4 alkoxy, C1-C4 alkoxycarbonyl,hydroxyl, (R⁵)₃SiO—, (R⁵)₂N—, —COOH, (R⁵)₂N—CH₂— or phenyl, it beingpossible for any two adjacent radicals from the group A, B, C, D and Eto form a conjoint 5- or 6-membered saturated or unsaturated ring whichmay also include N, S or O as heteroatom;

F is hydrogen or methyl;

R² and R³ independently of one another or simultaneously are C1-C4alkyl, C2-C6 hydroxyalkyl (with the hydroxyl group in position 2relative to the quaternary nitrogen) or R¹;

R⁴ is hydrogen, methyl, C2-C10 alkyl, C3-C8 cycloalkyl or C2-C12 alkoxy;

R5 is C1-C4 alkyl;

Y⁻ is R⁶COO⁻ or OH⁻; and

R⁶ is hydrogen or a branched or unbranched aliphatic or araliphaticC1-C10 alkyl radical.

The invention further preferably provides a trimerization catalyst forpreparing color-reduced polyisocyanates containing isocyanurate groups,of the general formula (I)

where

and where the variables are defined as follows:

A, B, C, D and E independently of one another or simultaneously arehydrogen, chioro, C1-C4 alkyl, C1-C4 alkoxy, C1-C4 alkoxycarbonyl,hydroxyl, (R⁵)₃SiO—, (R⁵)₂N—, —COOH, (R⁵)₂N—CH₂— or phenyl, it beingpossible for any two adjacent radicals from the group A, B, C, D and Eto form a conjoint 5- or 6-membered saturated or unsaturated ring whichmay also include N, S or O as heteroatom;

F is hydrogen or methyl;

R²and R³independently of one another or simultaneously are C1-C4 alkyl,C2-C6 hydroxyalkyl (with the hydroxyl group in position 2 relative tothe quaternary nitrogen) or R¹;

R⁴ is hydrogen, methyl, C2-C10 alkyl, C3-C8 cycloalkyl or C2-C12 alkoxy;

R⁵ is C1-C4 alkyl;

Y⁻ is R⁶COO⁻ or OH⁻; and

R⁶ is hydrogen or a branched or unbranched aliphatic or araliphaticC1-C10 alkyl radical.

The trimerization catalysts of the invention can be used to reactdiisocyanates prepared by the phosgene process or by a phosgene-freeprocess: for example, by thermal cleavage of (cyclo)aliphaticbiscarbamic esters (cf., e.g., EP-A-0 126 299 (U.S. Pat. No.4,596,678)). Preferable suitable starting diisocyanates for the processof the invention include aliphatic, cycloaliphatic and/or(cyclo)aliphatic diisocyanates, more preferable examples being1,4-diisocyanato-cyclohexane, 1,6-diisocyanatohexane (HDI),1,12-diisocyanatododecane, 1-isocyanato-3,3,5-trimethylcyclo-hexane(IPDI), 4,4′-diisocyanatodicyclohexylmethane,1,5-diisocyanato-2,2-dimethylpentane,1,5-diisocyanato-2-ethyl-2-propylpentane,1,5-diisocyanato-2-butyl-2-ethylpentane,1,6-diisocyanato-2,4,4-trimethylhexane and1,6-diisocyanato-2,4,4-trimethylhexane (TMDI),1,5-diisocyanato-2-methylpentane (MPDI), and2,5(2,6)-bis(isocyanatomethyl)bicyclo{2.2.1}heptane (NBDI). Even morepreference is given to the use of HDI, IPDI, MPDI, TMDI and NBDI.Mixtures of diisocyanates are possible.

The preparation of the polyisocyanates containing isocyanurate groups bypartial trimerization can take place either continuously (pipe reactoror reactor cascade) or batchwise. The catalysts of the invention arepreferably used in a low concentration of between 0.04 and 2.0% byweight. The exact amount depends on the individual catalyst, on thetarget conversion and on the procedure, the use of stirred reactorshaving been found from experience to necessitate the metered addition ofa larger amount of catalyst.

Under these conditions the trimerization can be carried out within 1-60minutes. The resulting compounds have one or more isocyanurate rings.Compounds with a uretdione structure may also be found as a secondarycomponent in small amounts. Compounds of this kind have been describedin the literature.

The trimerization catalysts of the invention can be prepared bypreferably reacting tertiary amines with a carboxylic acid and anoxirane. The molar ratio, with respect of the functionalities of thereactants, should be approximately 1:1:1; an excess of 20 mol % beingwith no significantly deleterious effect on catalyst performance,irrespective of the choice of the excess components. The reactiontemperature is preferably between 10° C. and 80° C., more preferablybetween 20° C. and 50° C. The reaction can be conducted in the presenceor absence of solvents. Examples of preferred solvents are ethyleneglycol, tetrahydrofuran, 1-butanol, methanol, and benzyl alcohol.

Preferable examples of tertiary amines suitable in principle areN,N-dimethyl-2-methoxybenzylamine, N,N-dimethyl-3-methoxybenzylamine,N,N-dimethyl-4-methoxy-benzylamine,N,N-dimethyl-2,3-dimethoxybenzylamine,N,N-dimethyl-3,4-dimethoxybenzylamine,N,N-dimethyl-3,5-dimethoxybenzylamine, N,N-dimethylbenzylamine,N,N-di-ethylbenzylamine-4-carboxylic acid,4-methoxycarbonyl-N,N-dimethylbenzylamine,4-ethoxycarbonyl-N,N-dimethylenzylamine,3-(N,N-di-ethylminomethyl)-N,N-dimethylenzamine,2-phenylthyidimethylamine, 1-phenylethyldiethylamine,4-hydroxy-N,N-dimethylbenzylamine,4-trimethylsiloxy-N,N-dimethylbenzylamine and N,N-dimethyinaphthylamine.

Preferable examples of suitable carboxylic acids are pivalic acid,hexanoic acid, acetic acid, 2-ethylhexanoic acid, propanoic acid, adipicacid, succinic acid and oleic acid.

Preferable oxiranes are aliphatic or araliphatic compounds having1,2-epoxide groups; that is, for example, ethylene oxide, propyleneoxide, 1,2-butylene oxide, 1,2-dodecene oxide, and 2,2-dimethyloxirane.The oxiranes can also be functionalized epoxy compounds such as, forexample, 2,3-epoxypropyl isopropyl ether.

In order to prepare polyisocyanates containing isocyanurate groups, thecatalysts of the invention are used preferably in small amounts. Theexact amount can readily be determined by experiment and depends on thecatalytic activity of the individual catalyst, on the target conversionand on the procedure.

In accordance with the invention, the hydroxyalkylammonium carboxylatesof the formula I are preferably used in an amount of 0.04-2% by weight,more preferably 0.06-1.5% by weight, and even more preferably between0.08 and 1.4% by weight based on the weight of the (cyclo)aliphaticdiisocyanate used.

The process of the invention is preferably conducted at temperatures ofbetween 35° C. and 185° C. Below 35° C., the amount of catalyst requiredfor the trimerization has been found from experience to be so great thatcolor problems may result. At temperatures above 185° C., there maylikewise be unwanted discoloration of the polyisocyanates containingisocyanurate groups. Trimerization preferably takes place in thepresence of the catalysts of the invention at temperatures between 50°C. and 175° C.

In accordance with the invention, the trimerization of the diisocyanatesis carried out either batchwise or continuously.

In the case of the batch process, a stirred reactor is preferably used.In this case, the mixture of diisocyanate and catalyst are charged tothe reactor usually at room temperature. Subsequently, the temperatureof the reaction mixture is raised to 35-140° C., and preferably to50-110° C., in order to initiate the trimerization. Alternatively, thecatalyst can be metered in after the diisocyanate has reached thenecessary temperature for the reaction. The trimerization is exothermic,and the catalyst is destroyed in the course of the reaction.

Continuous trimerization is preferably conducted in a reaction coil withcontinuous, simultaneous metered addition of the diisocyanate and of thetrimerization catalyst at from 40 to 120° C. over a period of from 1 to7 minutes. In a reaction pipe with a small diameter, high flow rate dareachieved. Furthermore, it is very advantageous to heat thediisocyanate/catalyst mixture to about 50 to 60° C. before entry intothe reaction pipe. An especially important factor is the meteredaddition of the catalyst. It is particularly preferable to mix thestarting materials thoroughly prior to entry into the reaction coil. Formore precise metering with small amounts of catalyst, and in order togenerate a better quality of thorough mixing, it can be advantageous todissolve the catalyst in an appropriate organic solvent. Appropriatesolvents are in principle those in which the catalyst is readilysoluble. Preferably, however, the use of solvents is very largelydispensed with.

The temperature of the sections of the reaction coil is judiciouslychosen such that the preheat zone is at about 40 to 60° C., the reactionzone at from 70 to 120° C., preferably from 70 to 100° C., and thecooling zone at from 20 to 40° C. The optimum temperature conditionsmust in each case be adapted to the required conditions for thediisocyanate to be trimerized.

In order to remove unreacted diisocyanate, the reaction mixture issubjected to flash evaporation or short path distillation.

Preferable starting compounds appropriate for the trimerization arediisocyanates having aliphatic, cycloaliphatic or aliphatic andcycloaliphatic isocyanate groups which have been prepared by thephosgene process or by a phosgene-free process, or else mixtures of suchdiisocyanates. Suitable aliphatic diisocyanates have preferably 3 to 16,with particular preference 4 to 12, carbon atoms in their linear orbranched alkylene substructure. Suitable low-chlorine cycloaliphaticdiisocyanates have preferably 4 to 18, with particular preference 6 to15, carbon atoms in their cycloalkylene substructure. Specific examplesthat may be mentioned include1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI),1,6-hexamethylene diisocyanate (HDI), 2-methyl-1,5-pentamethylenediisocyanate (MPDI), 2,5(2,6)-bis(isocyanatomethyl)bicyclo{2.2.1}heptane(NBDI), and 2,2,4- or 2,4,4-trimethyl-1,6-hexamethylene diisocyanate(TMDI).

The monomer-freed isocyanurates containing isocyanurate groups andprepared in accordance with the invention represent useful intermediatesfor polyurethane coatings, such as leather coatings and textilecoatings, and for polyurethane dispersions and adhesives, and areparticularly valuable as polyisocyanate components in 1- and 2-componentpolyurethane systems for weather- and light-stable polyurethane coatingmaterials. In these applications, the process products of the inventioncan be used either as such or else in a form in which they are blockedwith blocking agents. Examples of suitable blocking agents in that caseare lactams such as ε-caprolactam, oximes such as methyl ethyl ketoximeor butanone oxime, triazoles such as 1H-1,2,4-triazole, readilyenolizable compounds such as acetoacetic esters or acetylacetone, orelse malonic acid derivatives such as malonic diesters having 1-10carbon atoms in the alcohol residues.

EXAMPLES

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

A. Preparation of the Catalysts

All reactions are carried out under an inert gas atmosphere, preferablyunder nitrogen.

A.1. Preparation ofN-(2-hydroxypropyl)-N,N-dimethyl-N-(3-methoxybenzyl)ammonium2-ethylhexanoate (Cat. 1)

A three-necked flask with Claisen attachment, reflux condenser, internalthermometer, mechanical stirrer attachment, dropping funnel, gas inletand gas outlet is charged with 3-methoxybenzyldimethylamine (1 mol),which is admixed at room temperature with propylene oxide (1 mol;conditioned to about 0° C.), with stirring. Subsequently,2-ethylhexanoic acid (1 mol) is metered in at a rate such that thetemperature of the reaction solution does not exceed 35° C. Whenaddition is complete, stirring is continued at room temperature. Thecatalyst attains its full activity after about 2 to 3 days.

A.2. Preparation ofN-(2-hydroxybutyl)-N,N-dimethyl-N-(3-methoxybenzyl)ammonium2-ethylhexanoate (Cat. 2)

A three-necked flask with Claisen attachment, reflux condenser, internalthermometer, mechanical stirrer attachment, dropping funnel, gas inletand gas outlet is charged with 3-methoxybenzyldimethylamine (1 mol)which is admixed at 40° C. with 1,2-butene oxide (1 mol), with stirring.Subsequently, 2-ethylhexanoic acid (1 mol) is metered in at a rate suchthat the temperature of the reaction solution does not exceed 50° C.When addition is complete, stirring is continued at room temperature.The catalyst attains its full activity after about 2 days.

A.3. Preparation ofN-(2-hydroxybutyl)-N,N-dimethyl-N-(3-methoxybenzyl)ammonium pivalate(Cat. 3)

The catalyst is prepared as for Cat. 2 (A.2.). Pivalic acid-is usedinstead of 2-ethylhexanoate. The catalyst obtains its full activityafter about 2 days.

A.4. Preparation ofN-(2-hydroxypropyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammonium2-ethylhexanoate (Cat. 4)

The catalyst is prepared as for Cat. 1 (A.1.).3,4-dimethoxybenzyldimethylamine is used instead of3-methoxybenzyldimethylamine. The catalyst obtains its full activityafter about 2 to 3 days.

A.5. Preparation ofN-(2-hydroxypropyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammonium formate(Cat. 5)

A three-necked flask with Claisen attachment, reflux condenser, internalthermometer, mechanical stirrer attachment, dropping funnel, gas inletand gas outlet is charged with 3,4-dimethoxybenzyldimethylamine (1 mol)which is admixed at room temperature with propylene oxide (1 mol;conditioned to about 0° C.), with stirring. Subsequently, formic acid (1mol) is metered in at a rate such that the temperature of the reactionsolution does not exceed 35° C. When addition is complete, stirring iscontinued at room temperature. The catalyst attains its full activityafter about 2 to 3 days.

A.6. Preparation ofN-(2-hydroxybutyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammonium2-ethylhexanoate (Cat. 6)

In a three-necked flask with Claisen attachment, reflux condenser,internal thermometer, mechanical stirrer attachment, dropping funnel, aswell as gas inlet and gas outlet 3,4-dimethoxybenzyldimethylamine (1mol) is admixed at 40° C. with 1,2-butene oxide (1 mol), with stirring.Subsequently, 2-ethylhexanoic acid (1 mol) is metered in at a rate suchthat the temperature of the reaction solution does not exceed 50° C.When addition is complete, stirring is continued at room temperature.The catalyst attains its full activity after about 2 days.

A.7. Preparation ofN-(2-hydroxyhexyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammonium2-ethylhexanoate (Cat. 7)

The catalyst is prepared as for Cat. 6 (A.6.). 1,2-hexene oxide is usedinstead of 1,2-butene oxide. The catalyst obtains its full activityafter about 2 days.

A.8. Preparation ofN-(2-hydroxyhexyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammoniumhexanoate (Cat. 8)

In a three-necked flask with Claisen attachment, reflux condenser,internal thermometer, mechanical stirrer attachment, dropping funnel, aswell as gas inlet and gas outlet 3-methoxybenzyldimethylamine (1 mol) isadmixed at 40° C. with 1,2-hexene oxide (1 mol), with stirring.Subsequently, hexanoic acid (1 mol) is metered in at a rate such thatthe temperature of the reaction solution does not exceed 50° C. Whenaddition is complete, stirring is continued at room temperature. Thecatalyst attains its full activity after about 2 days.

A.9. Preparation ofN-(2-hydroxybutyl)-N,N-dimethyl-N-(3-methylbenzyl)ammonium2-ethylhexanoate (Cat. 9)

The catalyst is prepared as for Cat. 2 (A.2.).3-methylbenzyldimethylamine is used instead of3-methoxybenzyldimethylamine. The catalyst attains its full activityafter about 2 days.

A.10. Preparation ofN-(2-hydroxybutyl)-N,N-dimethyl-N-(2-methoxybenzyl)ammonium2-ethylhexanoate (Cat. 10)

The catalyst is prepared as for Cat. 2 (A.2.).2-methoxybenzyldimethylamine is used instead of3-methoxybenzyldimethylamine. The catalyst attains its full activityafter about 2 days.

Extractive purification of the catalysts with hexane (5×10 ml) ispossible but in general has no significantly positive effect on thecolor quality of the monomer-freed polyisocyanates.

B. Trimerization Experiments: Examples 1-20 and Comparative Examples 1-5

B. 1. Trimerization of the isocyanates

Catalyst and (cyclo)aliphatic diisocyanate are introduced into thereactor at room temperature. The temperature of the mechanically stirredreaction mixture, which is maintained under an inert gas atmosphere(N₂), is raised continuously over the course of from 10 to 12 min to thestart temperature (about 70° C.). Following initiation of the exothermictrimerization reaction, the heat source is removed. The temperature ofthe reaction mixture passes through a maximum and falls off againfollowing the thermal deactivation of the catalyst, which takes place inthe course of the reaction. The reaction mixture is cooled to roomtemperature and the excess monomer is separated off from thepolyisocyanate by flash evaporation.

The comparison catalysts used wereN-(2-hydroxypropyl)-N,N,N-trimethylammonium 2-ethylhexanoate (C-Cat. 1),N-(2-hydroxypropyl)-N,N,N-trimethylammonium formate (C-Cat. 2), andN-(2-hydroxypropyl)-N,N,N-trimethyl-ammonium hydroxide (C-Cat. 3).

The results of the trimerization experiments are summarized in Table 1and underscore the performance of the catalysts of the invention incomparison to conventional trimerization catalysts of the choline type.The color numbers can be improved further if the trimerization isconducted continuously. In that case, the superiority of the catalystsof the invention as documented in Table 1 is retained. In principle,low-chlorine (chlorine content <100 mg/kg) and chlorine-free—prepared,for example, by the urea process (cf. e.g. EP-A-0 355443)—(cyclo)aliphatic diisocyanates give the best results. Alcohols suchas benzyl alcohol or butanol can be used as cocatalysts or to dilute thecatalysts of the invention, with no deleterious effect on quality.

TABLE 1 Polyisocyanate Monomer- containing freed isocyanurate isocyan-Catalyst groups urates Amount NCO Color Color Diiso- (% by contentnumber number Entry cyanate Type wt) (% by wt.) (Hazen) (Hazen) Compara-tive Example  1 IPDI C-Cat.1 0.20 30.2 105 131⁽¹⁾  2 HDI C-Cat.1 0.1035.4 97 176  3 MPDI C-Cat.1 0.10 36.4 108 201  4 IPDI C-Cat.2 0.20 29.9178 212⁽¹⁾  5 IPDI C-Cat.3 0.20 29.4 98 131⁽¹⁾ Example  1 IPDI Cat.10.25 30.4 43 61⁽¹⁾  2 HDI Cat.1 0.10 38.8 23 49  3 IPDI Cat.2 0.25 30.542 62⁽¹⁾  4 HDI Cat.2 0.10 38.8 26 49  5 MPDI Cat.2 0.10 39.8 29 56  6IPDI Cat.3 0.25 30.5 47 71⁽¹⁾  7 HDI Cat.3 0.10 38.9 30 54  8 IPDI Cat.40.19 30.9 37 52⁽¹⁾  9 HDI Cat.4 0.10 38.7 27 52 10 IPDI Cat.5 0.19 31.061 98⁽¹⁾ 11 HDI Cat.5 0.10 39.1 38 91 12 IPDI Cat.6 0.19 30.9 36 50⁽¹⁾13 HDI Cat.6 0.10 38.8 28 50 14 IPDI Cat.7 0.19 30.8 32 45⁽¹⁾ 15 HDICat.7 0.10 38.7 22 43 16 MPDI Cat.7 0.10 39.9 19 41 17 IPDI Cat.8 0.1930.7 33 47⁽¹⁾ 18 HDI Cat.8 0.10 38.9 21 40 19 IPDI Cat.9 0.35 30.4 4260⁽¹⁾ 20 HDI Cat.9 0.10 38.9 25 49 21 IPDI Cat.10 0.25 30.4 40 57⁽¹⁾ 22HDI Cat.10 0.10 38.8 28 52 23 IPDI⁽³⁾ Cat.2⁽²⁾ 0.25 30.4 44 63⁽¹⁾ 24IPDI Cat.2 0.25 30.6 27 35⁽¹⁾ 25 IPDI Cat.7⁽²⁾ 0.19 30.7 32 46⁽¹⁾ 26IDPI⁽⁴⁾ Cat.7 0.19 30.8 28 39⁽¹⁾ ⁽¹⁾50% strength solution of butylacetate; ⁽²⁾Cat.2 as a 90% strength in benzyl alcohol; ⁽³⁾IPDI withchlorine content <100 ppm; ⁽⁴⁾chlorine-free IPDI (prepared by the ureaprocess).

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

The entire contents of each of the abovementioned references, patentsand published applications is hereby incorporated by reference, the sameas if set forth at length.

This application is based on German patent application 19944373.4, filedSep. 16, 1999, and incorporated herein by reference in its entirety.

What is claimed is:
 1. A process for preparing color-reducedpolyisocyanates containing isocyanurate groups, comprising: trimerizingat least one diisocyanate in the presence of 0.04-2% by weight, based onweight of said diisocyanate, at least one trimerization catalyst of theformula (I):

 wherein

 and wherein: A, B, C, D and E independently of one another orsimultaneously are hydrogen, chloro, C1-C4 alkyl, C1-C4 alkoxy, C1-C4alkoxycarbonyl, hydroxyl, or phenyl; F is hydrogen or methyl; R² and R³independently of one another or simultaneously are C1-C4 alkyl, C2-C6hydroxyalkyl with the hydroxyl group in position 2 relative to thequaternary nitrogen or R¹; R⁴ is hydrogen, methyl, C2-C10 alkyl, C3-C8cycloalkyl or C2-C12 alkoxy; R⁵ is C1-C4 alkyl; Y⁻ is R⁶COO⁻ or OH⁻; andR⁶ is hydrogen or a branched or unbranched, aliphatic or araliphatic,C1-C10 alkyl radical.
 2. The process as claimed in claim 1, wherein saidtrimerizing is conducted at a temperature between 35° C. and 185° C. 3.The process as claimed in claim 1, wherein said diisocyanate is adiisocyanate selected from the group consisting of aliphatic,cycloaliphatic, (cyclo)aliphatic, and mixtures thereof.
 4. The processas claimed in claim 1, wherein said diisocyanate is selected from thegroup consisting of1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI),1,6-hexamethylene diisocyanate (HDI), 2-methyl-1,5-diisocyanate (MPDI),2,5-(2,6-)-bis(isocyanatomethyl)bicyclo{2.2.1}heptane (NBDI),2,2,4-trimethyl-1,6-hexamethylene diisocyanate,2,4,4-trimethyl-1,6-hexamethylene diisocyanate (TMDI) and mixturesthereof.
 5. The process as claimed in claim 1, wherein said catalyst ispresent in an amount of 0.06-1.5% by weight.
 6. The process as claimedin claim 1, further comprising preparing said diisocyanate by a processselected from the group consisting of a phosgene process and aphosgene-free process.
 7. The process as claimed in claim 1, furthercomprising separating off an excess diisocyanate.
 8. The process asclaimed in claim 1, further comprising polymerizing said polyisocyanatesand producing a polyurethane.
 9. The process as claimed in claim 1,further comprising blocking said polyisocyanates with a blocking agentselected from the group consisting of lactams, ε-caprolactam, oximes,methyl ethyl ketoxime, butanone oxime, triazole, 1H-1,2,4-triazole,readily enolizable compounds, acetoacetic esters, acetylacetone, malonicacid derivatives, malonic diesters having 1-10 carbon atoms in thealcohol residues, and mixtures thereof.
 10. The process as claimed inclaim 1, wherein the catalyst is selected from the group consisting of:N-(2-hydroxypropyl)-N,N-dimethyl-N-(3-methoxybenzyl)ammonium2-ethylhexanoate;N-(2-hydroxybutyl)-N,N-dimethyl-N-(3-methoxybenzyl)ammonium2-ethylhexanoate;N-(2-hydroxybutyl)-N,N-dimethyl-N-(3-methoxybenzyl)ammonium pivalate;N-(2-hydroxypropyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammonium2-ethylhexanoate;N-(2-hydroxypropyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammoniumformate; N-(2-hydroxybutyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammonium2-ethylhexanoate;N-(2-hydroxyhexyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammonium2-ethylhexanoate;N-(2-hydroxyhexyl)-N,N-dimethyl-N-(3,4-dimethoxybenzyl)ammoniumhexanoate; N-(2-hydroxybutyl)-N,N-dimethyl-N-(3-methylbenzyl)ammonium2-ethylhexanoate;N-(2-hydroxybutyl)-N,N-dimethyl-N-(2-methoxybenzyl)ammonium2-ethylhexanoate.